Packet detection and timing synchronization for high performance wireless communications in substation automation

ABSTRACT

A method for packet detection in a wireless communication network employing time based scheduling of packets. The method is performed by a packet receiver in the wireless communication network. The method includes receiving a packet from a packet transmitter. The packet includes a preamble that is composed of a single orthogonal frequency-division multiplexing (OFDM) symbol and represented by a sequence of samples. T least part of the preamble is received within a packet detection window. Packet detection is performed in order to find a start of the packet only on those samples received within the packet detection window.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of InternationalApplication No. PCT/EP2018/053524, filed on Feb. 13, 2018, whichapplication is incorporated herein by reference.

TECHNICAL FIELD

Embodiments presented herein relate to a method, a packet receiver, acomputer program, and a computer program product for packet detection ina wireless communication network for power grid control.

BACKGROUND

Wireless networks to be used in the control of power grids, for examplein substation automation, require low latency and high reliability.Currently available industrial wireless standards, such as WirelessHART(where HART is short for Highway Addressable Remote Transducer) orWireless Network for Industrial Automation—Factory Automation (WIA-FA),are not able to provide very high performance in these regards, becausethey rely on non-optimized physical (PHY) communications layers. Forexample, WIA-FA is based on the IEEE 802.11g/n PHY layer, whose minimumtransmission time for a packet of 100 bits is around 30 μs, while manypower grid applications, currently based on wired local area networks(LANs) compliant with IEC 61850, require a slot time of a few μs or evenlower.

One cause of the long transmission time in IEEE 802.11 is the use oflong preamble sequences at the PHY layer. However, the long preamble inIEEE 802.11 is used for many purposes, including robust packet detectionand timing synchronization, which are crucial to ensure reliable messagedelivery. In this respect, packet detection generally refers to theprocess of approximately identifying the beginning of a packet, whiletiming synchronization generally refers to the process of finding theexact sample at which the useful part (such as the payload) of thepacket begins.

Existing schemes for packet detection and timing synchronization (e.g.as disclosed in U.S. Pat. No. 7,480,234 B1 and U.S. Pat. No. 7,280,621B1) rely on the presence of long repeated sequences in the packetpreamble, enabling the packet receiver to first correlate a knowntransmitted preamble with the received samples in order to detect thepacket, and then correlate the repeated parts to achieve precisesample-level synchronization. However, using a long preamble is notefficient when the packet size is short (e.g. as being the case in powergrid control applications) and thus fundamentally limits the achievablelatency.

Hence, there is still a need for improved packet detection in wirelesscommunication networks suitable for in the control of power grids.

SUMMARY

An object of embodiments herein is to provide efficient packet detectionthat does not suffer from the issues identified above, or at least wherethe issues noted above are reduced or mitigated.

According to a first aspect there is presented a method for packetdetection in a wireless communication network for power grid control.The wireless communication network employs time based scheduling ofpackets. The method is performed by a packet receiver in the wirelesscommunication network. The method comprises receiving a packet from apacket transmitter. The packet comprises a preamble. The preamble iscomposed of a single OFDM symbol and represented by a sequence ofsamples. At least part of the preamble is received within a packetdetection window. The method comprises performing packet detection inorder to find start of the packet only on those samples received withinthe packet detection window.

According to a second aspect there is presented a packet receiver forpacket detection in a wireless communication network for power gridcontrol. The wireless communication network employs time basedscheduling of packets. The packet receiver comprises processingcircuitry. The processing circuitry is configured to cause the packetreceiver to receive a packet from a packet transmitter. The preamble iscomposed of a single OFDM symbol and represented by a sequence ofsamples. At least part of the preamble is received within a packetdetection window. The processing circuitry is configured to cause thepacket receiver to perform packet detection in order to find start ofthe packet only on those samples received within the packet detectionwindow.

According to a third aspect there is presented a computer program forpacket detection in a wireless communication network for power gridcontrol, the computer program comprising computer program code which,when run on a packet receiver, causes the packet receiver to perform amethod according to the first aspect.

According to a fourth aspect there is presented a computer programproduct comprising a computer program according to the third aspect anda computer readable storage medium on which the computer program isstored. The computer readable storage medium could be a non-transitorycomputer readable storage medium.

Advantageously this provides efficient packet detection.

Advantageously, the proposed packet detection does not suffer from theissues noted above.

Advantageously, the proposed method allows an efficient packetstructure, enabling low latency wireless communications.

Indeed, reducing the preamble duration from five OFDM symbols (as inIEEE 802.11g) to just one OFDM symbol allows a reduction of nearly fivetimes in transmission time for too bits packets, achieving atransmission latency similar to wired communication networks.

Advantageously, the proposed method allows for robust packet detectionand timing synchronization to be performed also when the preamble isshort.

Advantageously the use of the packet detection window allows the packetdetection to be disabled when not needed, thus saving energy.

It is to be noted that any feature of the first, second, third, andfourth aspects may be applied to any other aspect, wherever appropriate.Likewise, any advantage of the first aspect may equally apply to thesecond, third, and/or fourth aspect, respectively, and vice versa. Otherobjectives, features and advantages of the enclosed embodiments will beapparent from the following detailed disclosure, from the attacheddependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted accordingto their ordinary meaning in the technical field, unless explicitlydefined otherwise herein. All references to “a/an/the element,apparatus, component, means, module, step, etc.” are to be interpretedopenly as referring to at least one instance of the element, apparatus,component, means, module, step, etc., unless explicitly statedotherwise. The steps of any method disclosed herein do not have to beperformed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a wireless communicationnetwork according to embodiments;

FIG. 2 schematically illustrates a packet receiver according to state ofthe art;

FIG. 3 schematically illustrates a packet structure according to stateof the art;

FIG. 4 is a flowchart of methods according to embodiments;

FIG. 5 is a schematic diagram showing functional modules of a packetreceiver according to an embodiment;

FIG. 6 schematically illustrates packet detection within a packetdetection window according to an embodiment;

FIG. 7 is a schematic diagram showing functional units of a packetreceiver according to an embodiment; and

FIG. 8 shows one example of a computer program product comprisingcomputer readable storage medium according to an embodiment.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter withreference to the accompanying drawings, in which certain embodiments ofthe inventive concept are shown. This inventive concept may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided by way of example so that this disclosure will be thorough andcomplete, and will fully convey the scope of the inventive concept tothose skilled in the art. Like numbers refer to like elements throughoutthe description. Any step or feature illustrated by dashed lines shouldbe regarded as optional.

FIG. 1 schematically illustrates a wireless communication network 100wherein the herein disclosed embodiments apply. Network entities denotednodes 200 a, 200 b, . . . , 200N are equipped with a radio frequency(RF) front-end that allows them to communicate over a wireless network110. Each node may represent a component of a substation automationsystem, such as a gateway, circuit breaker, circuit protector,transformer, switchgear, etc., that is configured for exchanging controlmessages.

Each node 200 a-200N may selectively act as a packet transmitter or apacket receiver. Without loss of generality it will hereinafter beassumed that node 200 a will act as a packet receiver and that any ofnodes 200 b-200N will act as a packet transmitter.

FIG. 2 schematically illustrates typical modules of a packet receiver200 a. The packet receiver 200 a of FIG. 2 comprises an automatic gaincontrol module, a packet detection module, a timing synchronizationmodule, a frequency synchronization module, a channel equalizationmodule, and a demodulation and decoding module. The functionality ofthese modules is as such known in the art and a description thereof istherefore omitted for brevity. In currently existing packet receivers200 a, these modules are implemented based on exploiting long repeatedsequences in the preamble of the received packets.

As an illustrative example, FIG. 3 schematically illustrates the packetstructure of a packet 300 used in IEEE 802.11g. In IEEE 802.11g thefirst short training sequences, a₁, a₂, . . . , a₇, of the legacy shorttraining field (L-STF) part of the PHY layer preamble are used forpacket detection, while the last ones, a₈, a₉, a₁₀, and the longtraining sequences, l₁, l₂, of the legacy long training field (L-LTF)part are used for coarse and fine timing synchronization respectively.

In order to achieve low latency for short-size packets exchanged inwireless networks for power grid control applications, the size of thePHY layer preamble should be kept small, possibly limited to just onesingle orthogonal frequency-division multiplexing (OFDM) symbol. Topreserve a good level of reliability, however, the packet receiver 200 amust still be able to perform its usual functions, including packetdetection and timing synchronization, using only this single OFDMsymbol.

The embodiments disclosed herein thus relate to mechanisms for packetdetection in a wireless communication network 100 for power gridcontrol. In order to obtain such mechanisms there is provided a packetreceiver 200 a, a method performed by the packet receiver 200 a, acomputer program product comprising code, for example in the form of acomputer program, that when run on a packet receiver 200 a, causes thepacket receiver 200 a to perform the method.

To achieve low latency the packet structure is optimized and a shortpreamble is used. Further, in order to ensure reliable communications,knowledge of the packet scheduling is used by a start-of-packetprediction mechanism that allows simple and reliable packet detectionand timing synchronization, even when a short preamble is adopted.

FIG. 4 is a flowchart illustrating embodiments of methods for packetdetection in a wireless communication network 100 for power gridcontrol.

The wireless communication network 100 employs time based scheduling ofpackets. The methods are performed by the packet receiver 200 a. Themethods are advantageously provided as computer programs 820.

It is assumed that the node acting as packet receiver 200 a receives apacket 600 from one of the other nodes acting as packet transmitter 200b-200N. The packet receiver 200 a is thus configured to perform stepS102:

S102: The packet receiver 200 a receives a packet 600 from a packettransmitter 200 b-200N.

The packet 600 comprises a preamble 610. The preamble 610 is composed ofa single OFDM symbol and represented by a sequence of samples. In someaspects the single OFDM symbol has a duration that corresponds to thefirst five L-STF short sequences in FIG. 3 .

At least part of the preamble 610 is received within a packet detectionwindow 630. Indeed, in wireless communication networks used for controlapplications, unlike in traditional communication networks, the channelaccess is regulated through time-slotted scheduling policies (e.g.time-division multiple access (TDMA)) to ensure determinism and avoidcollisions. In this way, each node (acting as a packet receiver 200 a)in the wireless communication network 100 knows that it can receivepackets only during predefined time slots. This fact is exploited by thepacket receiver 200 a to only receive packets within the packetdetection window 630.

The packet receiver 200 a then performs packet detection. Particularly,the packet receiver 200 a is configured to perform step S104:

S104: The packet receiver 200 a performs packet detection in order tofind start 640′ of the packet 600. The packet detection is performedonly on those samples that are received within the packet detectionwindow 630.

Advantageously, this enables simultaneous packet detection and timingsynchronization. As disclosed above, packet detection generally refersto the process of approximately identifying the beginning of a(received) packet 600 and timing synchronization generally refers to theprocess of finding the exact sample at which the useful part (such asthe payload) of the packet 600 begins.

Embodiments relating to further details of packet detection in awireless communication network 100 for power grid control as performedby the packet receiver 200 a will now be disclosed.

Parallel reference is made to FIG. 5 showing functional modules of thepacket receiver 200 a for packet detection and timing synchronizationaccording to an embodiment

The packet receiver 200 a in FIG. 5 comprises a start-of-packetprediction module 510. The start-of-packet prediction module 510 isconfigured to selectively enable and disable the detection of packets610, and hence when to open and close the packet detection window 630.Further aspects of the start-of-packet prediction module 510 will bedisclosed below.

There may be different ways to perform the packet detection in stepS104. Different embodiments relating thereto will now be described inturn.

In some aspects the packet detection in step S104 is based on comparingthose samples received within the packet detection window 630 with adefault sequence. Particularly, according to an embodiment performingpacket detection involves determining a similarity measure between arepresentation of those samples received within the packet detectionwindow 630 and a default normalized test sequence. In the example ofFIG. 5 the similarity measure is determined by the differentialdetection module 520.

There could be different ways to derive the representation of thesamples from the samples themselves.

The packet receiver 200 a in FIG. 5 comprises a delay and multiplymodule 530. The delay and multiply module 530 is configured to create aone-sample delayed copy of the received sequence and multiply thisone-sample delayed copy with the original received sequence through aHadamard product.

Particularly, according to an embodiment the samples received within thepacket detection window 630 defines a test sequence. The packet receiver200 a is then configured to perform (optional) step S104 a as part ofperforming the packet detection in step S104:

S104 a: The packet receiver 200 a multiplies the test sequence with aone-sample delayed copy of itself, resulting in a multiplied testsequence.

In this way the impact of frequency offsets in the detection performanceis minimized.

The packet receiver 200 a in FIG. 5 comprises a normalize module 540.The normalize module 54 o is configured to normalize the multiplied testsequence with respect to its average power. Thus, according to anembodiment the packet receiver 200 a is configured to perform (optional)step S104 b as part of performing the packet detection in step S104:

S104 b: The packet receiver 200 a normalizes the multiplied testsequence with respect to its total power, resulting in a normalized testsequence.

In this way the detection process is independent on the receiving power.

The packet receiver 200 a in FIG. 5 comprises a correlate module 550.The correlate module 550 is configured to compare the normalized testsequence to a default sequence. According to an embodiment the packetreceiver 200 a is thus configured to perform (optional) step S104 c aspart of performing the packet detection in step S104:

S104 c: The packet receiver 200 a correlates the normalized testsequence with a default normalized test sequence, resulting in acorrelated test sequence.

The representation of those samples received within the packet detectionwindow 630 is thus defined by the normalized test sequence.

There could be different examples of default normalized test sequences.According to an embodiment the default normalized test sequence is adefault preamble sequence (also multiplied by its one-sample delayedversion and normalized).

The packet receiver 200 a in FIG. 5 comprises a find maximum module 560.The find maximum module 560 is configured to find the maximum value ofthe correlated test sequence. Particularly, according to an embodimentthe packet receiver 200 a is configured to perform (optional) step S104d as part of performing the packet detection in step S104:

S104 d: The packet receiver 200 a identifies the sample in the testsequence for which the correlated test sequence has its maximum value.The sample is to then determined to define the start 640′ of the packet600.

This enables the precise sample at which the packet 600 starts to befound.

In some aspects the start 640′ of the packet 600 is only successfullyidentified when the maximum value of the correlated test sequenceexceeds a specified packet detection threshold value Δ. Therefore,according to an embodiment the packet receiver 200 a is configured toperform (optional) step S104 e as part of performing the packetdetection in step S104:

S104 e: The packet receiver 200 a compares the maximum value to a packetdetection threshold value Δ. The sample is then determined to define thestart 640′ of the packet 600 only when the maximum value exceeds thepacket detection threshold value Δ. In some aspects the value of Δdepends on the expected signal to noise ratio (SNR) at the packetreceiver 200 a and/or the length of the preamble 610. The SNR might, forexample, be determined based on the transmission bandwidth, thetransmission power and the link distance. For each SNR and preamblelength, an optimal packet detection threshold value Δ can be obtainedvia theoretical analysis or simulations.

Further aspects of the packet detection window 630 and thestart-of-packet prediction module 510 will now be disclosed.

In some aspects the packet detection window 630 is centered on theexpected start instant 640 of the received packet 600, as shown in FIG.6 . The packet 600 comprises a preamble 610 and a data part 620. Asdisclosed above, packet detection is enabled only during this window.According to an embodiment the packet detection window 630 is openedaccording to the time based scheduling. A packet detection window 630 oftwo or more samples is considered rather than a single sample, becausethe actual arrival time of the packet 600 can be slightly delayed oranticipated with respect to the expected one due to synchronizationmismatches between the packet receiver 200 a and the packet transmitter200 b-200N, as illustrated in FIG. 6 .

The duration of the packet detection window 630 is dimensioned to ensurethat the maximum deviation between the expected arrival time (as definedby the start instant 640) and the actual arrival time (as defined by thestart 640) of the packet 600 lies within the packet detection window630.

The expected arrival time of the packet 600 can be derived based on thenominal distance, d₀, between the packet transmitter 200 b-200N and thepacket receiver 200 a. The actual arrival time depends on the actualdistance, d, between the packet transmitter 200 b-200N and the packetreceiver 200 a. The maximum absolute difference between d and d₀, whichis defined by d_(max) is strictly related to the maximum transmissionand reception range of the wireless communication network 100.

The duration of the packet detection window (in seconds) should be setto:

${T = \frac{2 \cdot d_{{ma}\; x}}{c}},$

where c=2.99792×10⁸ m/s is the speed of light.

According to an embodiment the packet detection window 630 has a lengthin time of between 100 ns to 200 ns, preferably between 125 ns and 175ns, most preferably 150 ns.

The duration, W, of the packet detection window 630 in samples generallydepends on the sampling interval, T_(s), at the packet receiver 200 aand can be determined as:

$W = {\left\lceil \frac{T}{T_{s}} \right\rceil.}$

As a non-limiting illustrative example, with a maximum distancedeviation of d_(max)=20 m and a sampling interval of T_(s)=50 ns, thepacket detection window has a length of T=133.4 ns, corresponding to W=3samples.

The use of the packet detection window 630 to enable/disable packetdetection allows a simpler decoding process and lower energyconsumption, since the packet receiver 200 a does not need tocontinuously correlate all the received samples but only those withinthe packet detection window 630.

Further, the use of the packet detection window 630 improves thereliability of the packet detection process. In more detail, since thepreamble 610 is short, the correlation determined in step S104 c isgenerally weaker with respect to typical correlations computed on longersequences (e.g. using the IEEE 802.11 preamble). For this reason,so-called “false alarms” can arise, in which a sequence of noisy samplesis erroneously identified as the beginning of a packet. The use of thepacket detection window 630 allows to considerably mitigate this issue,since detection is only performed on a window of samples during whichthe packet 600 is expected to arrive.

FIG. 7 schematically illustrates, in terms of a number of functionalunits, the components of a packet receiver 200 a according to anembodiment. Processing circuitry 210 is provided using any combinationof one or more of a suitable central processing unit (CPU),multiprocessor, microcontroller, digital signal processor (DSP), etc.,capable of executing software instructions stored in a computer programproduct 810 (as in FIG. 8 ), e.g. in the form of a storage medium 230.The processing circuitry 210 may further be provided as at least oneapplication specific integrated circuit (ASIC), or field programmablegate array (FPGA).

Particularly, the processing circuitry 210 is configured to cause thepacket receiver 200 a to perform a set of operations, or steps,S102-S104 e, as disclosed above. For example, the storage medium 230 maystore the set of operations, and the processing circuitry 210 may beconfigured to retrieve the set of operations from the storage medium 230to cause the packet receiver 200 a to perform the set of operations. Theset of operations may be provided as a set of executable instructions.

Thus the processing circuitry 210 is thereby arranged to execute methodsas herein disclosed. The storage medium 230 may also comprise persistentstorage, which, for example, can be any single one or combination ofmagnetic memory, optical memory, solid state memory or even remotelymounted memory. The packet receiver 200 a may further comprise acommunications interface 220 at least configured for communications withat least one packet transmitter 200 a-200N. As such the communicationsinterface 220 may comprise one or more transmitters and receivers,comprising analogue and digital components. The processing circuitry 210controls the general operation of the packet receiver 200 a e.g. bysending data and control signals to the communications interface 220 andthe storage medium 23 o, by receiving data and reports from thecommunications interface 220, and by retrieving data and instructionsfrom the storage medium 230. Other components, as well as the relatedfunctionality, of the packet receiver 200 a are omitted in order not toobscure the concepts presented herein.

FIG. 8 shows one example of a computer program product 810 comprisingcomputer readable storage medium 830. On this computer readable storagemedium 830, a computer program 820 can be stored, which computer program820 can cause the processing circuitry 210 and thereto operativelycoupled entities and devices, such as the communications interface 220and the storage medium 230, to execute methods according to embodimentsdescribed herein. The computer program 820 and/or computer programproduct 810 may thus provide means for performing any steps as hereindisclosed.

In the example of FIG. 8 , the computer program product 810 isillustrated as an optical disc, such as a CD (compact disc) or a DVD(digital versatile disc) or a Blu-Ray disc. The computer program product810 could also be embodied as a memory, such as a random access memory(RAM), a read-only memory (ROM), an erasable programmable read-onlymemory (EPROM), or an electrically erasable programmable read-onlymemory (EEPROM) and more particularly as a non-volatile storage mediumof a device in an external memory such as a USB (Universal Serial Bus)memory or a Flash memory, such as a compact Flash memory. Thus, whilethe computer program 820 is here schematically shown as a track on thedepicted optical disk, the computer program 820 can be stored in any waywhich is suitable for the computer program product 810.

The inventive concept has mainly been described above with reference toa few embodiments. However, as is readily appreciated by a personskilled in the art, other embodiments than the ones disclosed above areequally possible within the scope of the inventive concept, as definedby the appended patent claims.

The invention claimed is:
 1. A method for packet detection in a wirelesscommunication network employing time based scheduling of packets, themethod being performed by a packet receiver in the wirelesscommunication network, the method comprising: receiving a packet from apacket transmitter, wherein the packet comprises a preamble that iscomposed of a single orthogonal frequency-division multiplexing (OFDM)symbol and represented by a sequence of samples, wherein at least partof the preamble is received within a duration of a packet detectionwindow, and wherein the packet detection window is centered on anexpected arrival time of the packet; and performing packet detection tofind a start of the packet only on those samples received within theduration of the packet detection window.
 2. The method according toclaim 1, wherein performing packet detection comprises determining asimilarity measure between a representation of the samples receivedwithin the packet detection window and a default normalized testsequence.
 3. The method according to claim 2, wherein the defaultnormalized test sequence is a default preamble sequence.
 4. The methodaccording to claim 2 wherein the representation of the samples receivedwithin the packet detection window is defined by the default normalizedtest sequence.
 5. The method according to claim 1, wherein the samplesreceived within the packet detection window define a test sequence andwherein performing packet detection further comprises multiplying thetest sequence with a one-sample delayed copy of itself, resulting in amultiplied test sequence.
 6. The method according to claim 5, whereinperforming packet detection further comprises normalizing the multipliedtest sequence with respect to its total power, resulting in a normalizedtest sequence.
 7. The method according to claim 6, wherein performingpacket detection further comprises correlating the normalized testsequence with a default normalized test sequence, resulting in acorrelated test sequence.
 8. The method according to claim 7, whereinthe default normalized test sequence is a default preamble sequence. 9.The method according to claim 7, wherein the representation of thesamples received within the packet detection window is defined by thenormalized test sequence.
 10. The method according to claim 7, whereinperforming packet detection further comprises identifying a sample inthe test sequence for which the correlated test sequence has its maximumvalue, wherein the identified sample is determined to define the startof the packet.
 11. The method according to claim 7, wherein performingpacket detection further comprises: identifying a sample in the testsequence for which the correlated test sequence has its maximum value;and comparing the maximum value to a packet detection threshold value,wherein the identified sample is determined to define the start of thepacket only when the maximum value exceeds the packet detectionthreshold value.
 12. The method according to claim 1, wherein the packetdetection window is opened according to the time based scheduling. 13.The method according to claim 1, wherein the packet detection window hasa length in time of between 100 ns to 200 ns.
 14. The method accordingto claim 1, wherein the packet detection window has a length in time ofbetween 125 ns and 175 ns.
 15. The method according to claim 1, whereinthe packet receiver is part of a gateway, circuit breaker, circuitprotector, transformer, or switchgear.
 16. The method according to claim1, wherein the packet transmitter is part of a gateway, circuit breaker,circuit protector, transformer, or switchgear.
 17. The method accordingto claim 1, wherein the wireless communication network is associatedwith a power grid and wherein the packet carries information related tothe power grid.
 18. The method according to claim 1, wherein theduration of the packet detection window corresponds to a differencebetween an expected arrival time of the packet and an actual arrivaltime of the packet.
 19. A packet receiver for packet detection in awireless communication network employing time based scheduling ofpackets, the packet receiver comprising processing circuitry, theprocessing circuitry being configured to cause the packet receiver to:receive a packet from a packet transmitter, wherein the packet comprisesa preamble that is composed of a single orthogonal frequency-divisionmultiplexing (OFDM) symbol and represented by a sequence of samples andwherein at least part of the preamble is received within a duration of apacket detection window, the packet detection window being centered onan expected arrival time of the packet; and perform packet detection tofind a start of the packet only on those samples received within theduration of the packet detection window.
 20. The packet receiveraccording to claim 19, wherein the packet receiver is part of a gateway,circuit breaker, circuit protector, transformer, or switchgear.
 21. Thepacket receiver according to claim 19, wherein the duration of thepacket detection window corresponds to a difference between an expectedarrival time of the packet and an actual arrival time of the packet. 22.A non-transitory storage medium that stores a computer program forpacket detection in a wireless communication network that employs timebased scheduling of packets, the computer program comprising computercode which, when run on processing circuitry of a packet receiver,causes the packet receiver to: receive a packet from a packettransmitter, wherein the packet comprises a preamble, wherein thepreamble is composed of a single orthogonal frequency-divisionmultiplexing (OFDM) symbol and represented by a sequence of samples andwherein at least part of the preamble is received within a duration of apacket detection window, the packet detection window being centered onan expected arrival time of the packet; and perform packet detection inorder to find a start of the packet only on those samples receivedwithin the duration of a packet detection window.
 23. The non-transitorystorage medium of claim 22, wherein the duration of the packet detectionwindow corresponds to a difference between an expected arrival time ofthe packet and an actual arrival time of the packet.